The present invention relates generally to an apparatus and method of the creation of a virtual electrode. More particularly, the present invention relates to an apparatus and method of the creation of a virtual electrode, which is useful for the ablation of soft tissue and neoplasms.
The utilization of an electric current to produce an ameliorative effect on a bodily tissue has a long history, reportedly extending back to the ancient Greeks. The effects on bodily tissue from an applied electric current, and thus the dividing line between harmful and curative effects, will vary depending upon the voltage levels, current levels, the length of time the current is applied, and the tissue involved. One such effect resulting from the passage of an electric current through tissue is heat generation.
Body tissue, like all non-superconducting materials, conducts current with some degree of resistance. This resistance creates localized heating of the tissue through which the current is being conducted. The amount of heat generated will vary with the power P deposited in the tissue, which is a function of the product of the square of the current I and the resistance R of the tissue to the passage of the current through it (P=I2R.).
As current is applied to tissue, then, heat is generated due to the inherent resistance of the tissue. Deleterious effects in the cells making up the tissue begin to occur at about 42xc2x0 Celsius. As the temperature of the tissue increases because of the heat generated by the tissue""s resistance, the tissue will undergo profound changes and eventually, as the temperature becomes high enough, that is, generally greater than 45xc2x0 C., the cells will die. The zone of cell death is known as a lesion and the procedure followed to create the lesion is commonly called an ablation. As the temperature increases beyond cell death temperature, complete disintegration of the cell walls and cells caused by boiling off of the tissue""s water can occur. Cell death temperatures can vary somewhat with the type of tissue to which the power is being applied, but generally will begin to occur within the range of 45xc2x0 to 60xc2x0 C., though actual cell death of certain tissue cells may occur at a higher temperature.
In recent times, electric current has found advantageous use in surgery, with the development of a variety of surgical instruments for cutting tissue or for coagulating blood. Still more recently, the use of alternating electric current to ablate, that is, kill, various tissues has been explored. Typically, current having a frequency from about 3 kilohertz to about 300 gigahertz, which is generally known as radiofrequency or radiofrequency (RF) current, is used for this procedure. Destruction, that is, killing, of tissue using an RF current is commonly known as radiofrequency ablation. Often radiofrequency ablation is performed as a minimally invasive procedure and is thus known as radiofrequency catheter ablation because the procedure is performed through and with the use of a catheter. By way of example, radiofrequency catheter ablation has been used to ablate cardiac tissue responsible for irregular heartbeat arrhythmias.
The prior art applications of current to tissue have typically involved applying the current using a xe2x80x9cdryxe2x80x9d electrode. That is, a metal electrode is applied to the tissue desired to be affected and a generated electric current is passed through the electrode to the tissue. A commonly known example of an instrument having such an operating characteristic is an electrosurgical instrument known as a xe2x80x9cboviexe2x80x9d knife. This instrument includes a cutting/coagulating blade electrically attached to a current generator. The blade is applied to the tissue of a patient and the current passes through the blade into the tissue and through the patient""s body to a metal base electrode or ground plate usually placed underneath and in electrical contact with the patient. The base electrode is in turn electrically connected to the current generator so as to provide a complete circuit.
As the current from the bovie knife passes from the blade into the tissue, the resistance provided by the tissue creates heat. In the cutting mode, a sufficient application of power through the bovie to the tissue causes the fluid within the cell to turn to steam, creating a sufficient overpressure so as to burst the cell walls. The cells then dry up, desiccate, and carbonize, resulting in localized shrinking and an opening in the tissue. Alternatively, the bovie knife can be applied to bleeding vessels to heat and coagulate the blood flowing therefrom and thus stop the bleeding.
As previously noted, another use for electrical instruments in the treatment of the body is in the ablation of tissue. To expand further on the brief description given earlier of the ablation of cardiac tissue, it has long been known that a certain kind of heart tissue known as sino-atrial and atrio-ventricular nodes spontaneously generate an electrical signal that is propagated throughout the heart along conductive pathways to cause it to beat. Occasionally, certain heart tissue will xe2x80x9cmisfire,xe2x80x9d causing the heart to beat irregularly. If the errant electrical pathways can be determined, the tissue pathways can be ablated and the irregular heartbeat remedied. In such a procedure, an electrode is placed via a catheter into contact with the tissue and then current is applied to the tissue via the electrode from a generator of RF current. The applied current will cause the tissue in contact with the electrode to heat. Power will continue to be applied until the tissue reaches a temperature where the heart tissue dies, thereby destroying the errant electrical pathway and the cause of the irregular heartbeat.
Another procedure using RF ablation is transurethral needle ablation, or TUNA, which is used to create a lesion in the prostate gland for the treatment of benign prostatic hypertrophy (BPH) or the enlargement of the prostate gland. In a TUNA procedure, a needle having an exposed conductive tip is inserted into the prostate gland and current is applied to the prostate gland via the needle. As noted previously, the tissue of the prostate gland heats locally surrounding the needle tip as the current passes from the needle to the base electrode. A lesion is created as the tissue heats and the destroyed cells may be reabsorbed by the body, infiltrated with scar tissue, or just become non-functional.
While there are advantages and uses for such xe2x80x9cdryxe2x80x9d electrode instruments, there are also several notable disadvantages. One of these disadvantages is that during a procedure coagulumxe2x80x94dried blood cells and tissue cellsxe2x80x94will form on the electrode engaging the tissue. Coagulum acts as an insulator and effectively functions to prevent current transfer from the blade to the tissue. This coagulum xe2x80x9cinsulationxe2x80x9d can be overcome with more voltage so as to keep the current flowing, but only at the risk of arcing and injuring the patient. Thus, during surgery when the tissue is cut with an electrosurgical scalpel, a build-up of coagulated blood and desiccated tissue will occur on the blade, requiring the blade to be cleaned before further use. Typically, cleaning an electrode/scalpel used in this manner will involve simply scraping the dried tissue from the electrode/scalpel by rubbing the scalpel across an abrasive pad to remove the coagulum. This is a tedious procedure for the surgeon and the operating staff since it requires the xe2x80x9crealxe2x80x9d work of the surgery to be discontinued while the cleaning operation occurs. This procedure can be avoided with the use of specially coated blades that resist the build up of coagulum. Such specialty blades are costly, however.
A second disadvantage of the dry electrode approach is that the electrical heating of the tissue creates smoke that is now known to include cancer-causing agents. Thus, preferred uses of such equipment will include appropriate ventilation systems, which can themselves become quite elaborate and quite expensive.
A further, and perhaps the most significant, disadvantage of dry electrode electrosurgical tools is revealed during cardiac ablation procedures. During such a procedure, an electrode that is otherwise insulated but having an exposed, current carrying tip is inserted into the heart chamber and brought into contact with the inner or endocardial side of the heart wall where the ablation is to occur. The current is initiated and passes from the current generator to the needle tip electrode and from there into the tissue so that a lesion is created. Typically, however, the lesion created by a single insertion is insufficient to cure the irregular heartbeat because the lesion created is of an insufficient size to destroy the errant electrical pathway. Thus, multiple needle insertions and multiple current applications are almost always required to ablate the errant cardiac pathway, prolonging the surgery and thus increasing the potential risk to the patient.
This foregoing problem is also present in TUNA procedures, which similarly requires multiple insertions of the needle electrode into the prostate gland. Failing to do so will result in the failure to create a lesion of sufficient size such that the procedure produces a beneficial result. As with radiofrequency catheter ablation of cardiac tissue, then, the ability to create a lesion of the necessary size to alleviate BPH symptoms is limited and thus requires multiple insertions of the electrode into the prostate.
A typical lesion created with a dry electrode using RF current and a single insertion will normally not exceed one centimeter in diameter. This small sizexe2x80x94often too small to be of much or any therapeutic benefitxe2x80x94stems from the fact that the tissue surrounding the needle electrode tends to desiccate as the temperature of the tissue increases, leading to the creation of a high resistance to the further passage of current from the needle electrode into the tissue, all as previously noted with regard to the formation of coagulum on an electrosurgical scalpel. This high resistancexe2x80x94more properly termed impedance since typically an alternating current is being usedxe2x80x94between the needle electrode and the base electrode is commonly measured by the RF current generator. When the measured impedance reaches a pre-determined level, the generator will discontinue current generation. Discontinuance of the ablation procedure under these circumstances is necessary to avoid injury to the patient.
Thus, a typical procedure with a dry electrode may involve placing the needle electrode at a first desired location; energizing the electrode to ablate the tissue; continue applying current until the generator measures a high impedance and shuts down; moving the needle to a new location closely adjacent to the first location; and applying current again to the tissue through the needle electrode. This cycle of electrode placement, electrode energization, generator shut down, electrode re-emplacement, and electrode re-energization, will be continued until a lesion of the desired size has been created. As noted, this increases the length of the procedure for the patient. Additionally, multiple insertions increases the risk of at least one of the placements being in the wrong location and, consequently, the risk that healthy tissue may be undesirably affected while diseased treatment may be left untreated. The traditional RF ablation procedure of using a dry ablation therefore includes several patient risk factors that both patient and physician would prefer to reduce or eliminate.
The therapeutic advantages of RF current could be increased if a larger lesion could be created safely with a single positioning of the current-supplying electrode. A single positioning would allow the procedure to be carried out more expeditiously and more efficiently, reducing the time involved in the procedure. Larger lesions can be created in at least two ways. First, simply continuing to apply current to the patient with sufficiently increasing voltage to overcome the impedance rises will create a larger lesion, though almost always with undesirable results to the patient. Second, a larger lesion can be created if the current density, that is, the applied electrical energy, could be spread more efficiently throughout a larger volume of tissue. Spreading the current density over a larger tissue volume would correspondingly cause a larger volume of tissue to heat in the first instance. That is, by spreading the applied power throughout a larger tissue volume, the tissue would heat more uniformly over a larger volume, which would help to reduce the likelihood of generator shutdown due to high impedance conditions. The applied power, then, will cause the larger volume of tissue to be ablated safely, efficiently, and quickly.
Research conducted under the auspices of the assignee of the present invention has focused on spreading the current density throughout a larger tissue volume through the creation, maintenance, and control of a xe2x80x9cvirtual electrodexe2x80x9d within or adjacent to the tissue to be ablated. A virtual electrode can be created by the introduction of a conductive fluid, such as isotonic or hypertonic saline, into or onto the tissue to be ablated. The conductive fluid will facilitate the spread of the current density substantially equally throughout the extent of the flow of the conductive fluid, thus creating an electrodexe2x80x94a virtual electrodexe2x80x94substantially equal in extent to the size of the delivered conductive fluid. RF current can then be passed through the virtual electrode into the tissue.
A virtual electrode can be substantially larger in volume than the needle tip electrode typically used in RF interstitial ablation procedures and thus can create a larger lesion than can a dry, needle tip electrode. That is, the virtual electrode spreads or conducts the RF current density outward from the RF current sourcexe2x80x94such as a current carrying needle, forceps or other current delivery devicexe2x80x94into or onto a larger volume of tissue than is possible with instruments that rely on the use of a dry electrode. Stated otherwise, the creation of the virtual electrode enables the current to flow with reduced resistance or impedance throughout a larger volume of tissue, thus spreading the resistive heating created by the current flow through a larger volume of tissue and thereby creating a larger lesion than could otherwise be created with a dry electrode.
While the efficacy of RF current ablation techniques using a virtual electrode has been demonstrated in several studies, the currently available instruments useful in such procedures lags behind the research into and development of hoped-for useful treatment modalities for the ablation of soft tissue and malignancies. Thus, to perform current research procedures it is necessary to provide separately both a fluid pump and an RF current generator. The fluid pump requires an electrical connection and fluid conduits running from the fluid supply to the fluid delivery instrument. The generator also requires electrical connections and electric lines running from the generator to the surgical instrument, as well as a return electrical line from the ground plate when a monopolar electrode is used. Use of these systems is thus hampered by the many fluid and electrical lines surrounding the operating theater, all of which can easily become entangled and complicate their use.
Further, currently available generators provide limited control over the power application to the tissue. For example, such generators, which are often designed for cardiac ablation procedures, provide for an immediate or nearly immediate cessation of power upon the occurrence of equipment defined high impedance conditions. These generators are thus unable to operate continuouslyxe2x80x94and therefore unable to provide the most expeditious, efficient therapyxe2x80x94when operating in the presence of fleeting high impedance conditions. Consequently, the presently available generators will automatically shut off when such a condition occurs even though continued application of RF power would otherwise be safe. The generator must then be restarted, which is not only an unnecessary annoyance but also prolongs the procedure since such shutdowns can occur more than once during any one ablation procedure.
It would be desirable to have an apparatus and method capable of creating, maintaining, and controlling a virtual electrode while providing a controlled application of tissue ablating RF electric current to a tissue of interest so as to produce a lesion of desired size and configuration. Preferably, such an apparatus will be capable of adjusting the applied current and fluid flow in accord with the measured impedance of the tissue and/or the temperature of the tissue being ablated. It would also be desirable to have such an apparatus and method capable of continuing operation in the presence of transient high impedance conditions.
It is an object of the present invention to provide a new and improved apparatus and method that is not subject to the foregoing disadvantages.
It is another object of the present invention to provide an integrated conductive fluid supply and radio frequency current generator.
It is still another object of the present invention to provide a device having an integrally controlled and operated conductive fluid supply and radio frequency current generator that controls the applied power and rate of conductive fluid infusion in response to the measured impedance of the tissue being ablated.
It is yet another object of the present invention to provide an integrally controlled and operated conductive fluid supply and radio frequency current generator that controls the applied power and rate of conductive fluid infusion in response to the measured impedance transients.
It is still yet another object of the present invention to provide an integrally controlled and operated conductive fluid supply and radio frequency current generator that selectively controls the applied power and rate of conductive fluid infusion in response to the measured temperature of the tissue being ablated.
It is a further object of the present invention to provide a method for treating a patient with RF current via a virtual electrode to produce a lesion of a predetermined size.
The foregoing objects of the present invention are achieved by an apparatus and a method for producing a virtual electrode within or upon a tissue to be treated with radio frequency alternating electric current, such tissues including but not limited to liver, lung, cardiac, prostate, breast, and vascular tissues and neoplasms. An apparatus in accord with the present invention will include a supply of a conductive or electrolytic fluid to be provided to the patient, an alternating current generator, and a processor for creating, maintaining, and controlling the ablation process by the interstitial or surficial delivery of the fluid to a tissue and the delivery of electric power to the tissue via the virtual electrode.
The control of the virtual electrode and the ablation procedure is accomplished in response to measured temperatures at pre-determined distances from a current delivery device and/or measured impedances over pre-determined time intervals. The ablation procedure is adjusted due to transient impedance rises related to the novel apparatus and its operation as well as due to those impedance and temperature changes related to the flow of current from the virtual electrode through the tissue being treated. Such an apparatus in accord with the present invention will also preferably include a display for visualizing predetermined operational parameters and for visualizing the ongoing operation of the virtual electrode and the apparatus during an ablation procedure.
In addition, an apparatus in accord with the present invention will also preferably include an input apparatus such as a rotary encoder and mouse-type of device, a touch screen useful in the form of a menu-driven icon system, and/or a keyboard. The input apparatus is provided to allow the operator to input predetermined information, such as tissue type to be ablated, desired lesion size, type of electrolytic fluid being used, the particular surgical instrument being used, and any other desired parameter of interest.
An apparatus in accord with the present invention may also include one or more additional fluid administration systems providing a controlled delivery of at least a second fluid to the patient during an ablation procedure. Such a fluid may be provided for maintaining the temperature of the tissue surrounding the tissue to be ablated at a non-harmful temperature, such as by directly cooling the tissue. Alternatively, an insulating fluid such as dextrose may be infused into the tissue surrounding the tissue ablation site, thus increasing the impedance of this surrounding tissue to prevent current passage therethrough and thereby diminishing the likelihood of unwanted heating of this surrounding tissue.
A method in accord with the present invention will include the steps of delivering a conductive fluid to a predetermined tissue ablation site to be ablated for a predetermined time period that may range from zero seconds to about ten minutes, preferably within the range of zero to sixty seconds, thereby creating a virtual electrode, applying a predetermined power level of radio frequency current to the tissue via the virtual electrode, monitoring at least one of several parameters (including but not limited to impedance, tissue temperature at one or more pre-selected distances from the current delivery device, the applied power, and fluid flow), and adjusting either the applied power and/or the fluid flow in response to the measured parameters. The method may also include the placement of a current delivery device within the tissue ablation site, such as substantially at the center thereof, and/or the placement of at least one additional temperature sensor in tissue whose ablation is not desired yet is adjacent to the predetermined ablation site. By way of example only, where an increase of impedance of predetermined size occurs within a predetermined time period the applied power may be automatically reduced by a predetermined amount for a predetermined period of time. By way of yet another example, when a tissue target temperature is reached, the applied power may be reduced to maintain the temperature at the target temperature and reduce the likelihood of increasing the tissue temperature of the ablation site above the target temperature. By way of another example, the flow of conductive fluid to the tissue ablation site can be increased or decreased in response to one or more of the measured parameters (temperature, impedance, etc.).
The foregoing objects of the invention will become apparent to those skilled in the art when the following detailed description of the invention is read in conjunction with the accompanying drawings and claims. Throughout the drawings, like numerals refer to similar or identical parts.